[Unison-hackers] path option 'root testing' bug

[Matthew Richardson]

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I've encountered what looks like a well-reported 'bug' in unison, which I'm
interested in having resolved, but I don't have any ocaml expertise under my
belt, and thus am appealing to the list to try and get this looked at.

The problem is simple - if you specify a path of format:

path = a/b/c

Then folders a and b must exist on both source and target prior to unison
running - if this isn't true, then unison fails with the error:

"path a/b is not valid because a/b is not a directory in one of the replicas"

I've had a look at the code, and I believe I can see why this is occurring.  As
far as I can tell, the unison code treats path options as temporary moves of the
root to a new 'base'.  In this case it moves to a/b and then tries to sync from
there.  As neither of these exist, it fails.

The relevant code block is:

| Some(name, path') ->
    if not (isDir fspath here) then
      let error =
        if Path.isEmpty here then
(* error 1 *)
          Printf.sprintf
            "path %s is not valid because the root of one of the replicas \
             is not a directory"
            (Path.toString fullpath)
        else
(* error 2 *)
          Printf.sprintf
            "path %s is not valid because %s is not a directory in one of \
             the replicas"
            (Path.toString fullpath) (Path.toString here)

The test runs as follows:

Test if both paths are directories.

If path is 'empty' - error 1 reported.
Otherwise - error 2 reported.

I would suggest that the following might be a better test:

Test if both paths are directories.

If path is 'empty' - error 1 reported.
If target isn't a directory, try recursively making the path and repeating the
test - succeed or fail as appropriate.
If this 'mkdir -p' fails, report error 2.

This should allow the problem of the directories not existing to be resolved,
but should still fail if one replica has files, symlinks etc where a directory
is expected.

Of course, I might have completely misunderstood what unison is doing here (in
which case fell free to shoot me down), and I don't speak any ocaml to implement
this change (although if there's agreement this change would be useful I could
make the effort to learn if there isn't any available coding effort).

Comments?

Matthew
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